Westfield Group, the global mall operator, will buy the 50 percent stake in the retail space at the World Trade Center it does not already own from the Port Authority for $800 million.

The company paid $612.5 million for the other 50 percent in May 2012 and will make this payment to the Port within the next six weeks.

“We bought [the first half] a year ago and since then, the building has progressed, leasing interest and demand is stronger than it was at that time and both the value and the rents are more certain,” said Peter Lowy, co-CEO of Westfield. “It’s really in the timing so they got a better price.”

Asking rents are in the $500- to $600-per-square-foot range, sources said, comparable with some streets in SoHo. No tenants have yet been announced, but some may surface during the International Council of Shopping Centers meeting in New York next week.

Lowy declined to comment on rents or future tenants.

Westfield has also agreed to make a one-time payment to the Port within five years of its 2015 grand opening if Westfield gets higher rents.

The 365,000 square feet of WTC retail space covers stores now being marketed by Westfield inside the Calatrava-designed transportation hub and West Concourse as well as in the bases of and along the streets next to Larry Silverstein’s towers 3 and 4.

An additional 90,000 square feet will be sold to Westfield at an undisclosed price when Tower 2 is developed by Silverstein.

Separately, Silverstein and other defendants have been absolved of responsibility in the Sept. 11, 2001, destruction of the 7 World Trade Center building. A federal appeals court ruled Wednesday that negligence was not the cause of the collapse of the 47-story tower several hours after the twin towers were destroyed in the Sept. 11 terrorist attacks.